Advanced multimedia services continue to drive requirements for increasing data rates and higher performance in wireless systems. Current technologies for high performance communication systems, such as those specified by the Japanese integrated services digital broadcasting terrestrial standard (ISDB-T), employ communication methods based on Orthogonal Frequency Division Multiplexing (OFDM).
As known to those of skill in the art, multipath interference presents a significant impediment to effective wireless communication. Since a clear line-of-sight rarely exists between a transmitter and receiver, a signal typically gets reflected by buildings and other various obstacles. As a result, multiple versions of the signal travel along different paths before arriving at the receiver. Each path is subject to different conditions which can introduce phase shifts, time delays, attenuations, and other distortions that can destructively interfere with one another at the receiver. For example, the variable transmission delays can result in inter-symbol interference (ISI) when the different data signals arrive at the receiver simultaneously
In OFDM multiple sub-carrier systems, a higher rate data signal is divided among multiple narrowband sub-carriers that are orthogonal to one another in the frequency domain. Thus, the higher rate data signal is transmitted as a set of parallel lower rate data signals carried on separate sub-carriers
A received OFDM symbol in an OFDM system generally consists of both data and pilot synchronization information transmitted on the multiple sub-carriers multiplexed together and spanning multiple sample periods. Modulation and demodulation in an OFDM system uses an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver. At the transmitter, a cyclic prefix of a section of the IFFT output for each OFDM symbol is typically appended to the beginning of the OFDM symbol as a guard interval (GI). The length of the OFDM symbol before adding the guard interval is known as the useful symbol period duration At the receiver, the cyclic prefix is removed prior to the FFT demodulation by the appropriate positioning of an FFT window, with size equal to the useful symbol period duration, along a received sample sequence. The FFT demodulation transforms the window of received time domain samples, in the received sample sequence, to a frequency domain (OFDM) symbol.
A principle advantage of this type of communication system is that the lower data rate occupies a longer symbol period than in a higher rate single carrier system. The addition of the guard interval to each lower frequency symbol contains the dispersion caused by multipath within the longer symbol period, reducing ISI OFDM systems also offer a number of other advantages relevant to wireless applications, including high spectral efficiency and the ability to compensate for poor channel conditions, including signal fade.
A significant aspect of OFDM systems is the use of channel estimating techniques to correct for changes in the sub-carrier characteristics In pilot-based systems, a known symbol, or “pilot,” is transmitted at given sub-carrier frequencies and at given times. Since the receiver knows the transmitted symbol, any errors to the transmitted pilot due to sub-carrier conditions can be estimated and an appropriate correction calculated. Channel conditions for all sub-carriers and times can likewise be interpolated from the pilot information, allowing equalization of the signal and subsequent coherent demodulation
Further details regarding the design of OFDM systems can be found in co-pending, commonly-assigned U.S. patent application Ser. No. 12/272,629, filed Nov. 17, 2008, Ser. No. 12/277,247, filed Nov. 24, 2008, Ser. No. 12/277,258, filed Nov. 24, 2008, Ser. No. 12/365,726, filed Feb. 4, 2009, and Ser. No. 12/398,952, filed Mar. 5, 2009, all of which are hereby incorporated by reference in their entirety
Antenna diversity is a well recognized technique that generally employs two or more antennas to provide improved quality and reliability by reducing multipath interference. The multiple antennas give the receiver different observations of the same signal, each of which are subject to different conditions. Accordingly, even if one antenna is suffering from poor reception, an alternate antenna may still have good signal quality.
One class of existing diversity reception technologies includes systems that combine the signals received from each antenna. An example of this technique involves daisy-chaining receivers so that a multiple receivers associated with each antenna combines the signal with the others in series A more sophisticated system uses maximum ratio combining (MRC) to selectively weight and add the signals coherently to provide better results than simple addition Such methods are widely used in mobile and portable digital TV receivers However, any system that relies on combining signals must have duplicate a significant portion of the signal processing architecture for each antenna, depending upon the stage at which the signals are combined. Accordingly, such techniques offer good performance but at a substantial cost, typically almost a multiple equivalent to the number of antennas employed compared to a single antenna system.
Another strategy for diversity reception relies on switching between antennas based upon the quality of the signal being received. This technique offers appreciable cost savings but represents implementation and performance challenges, primarily due to loss of data and channel information during the switching process.
Thus, it would be desirable to provide a system and method of antenna diversity using antenna selection that offers increased performance. In particular, it would be desirable to provide antenna switching while minimizing disruption of data reception or channel estimation.